1. Field of the Invention
The present invention relates to a wireless local area network (WLAN), and more particularly, to a transmission opportunity truncation method in a WLAN.
2. Discussion of the Related Art
With the recent development of information communication technology, a variety of wireless communication techniques have been developed. Among these, there is a wireless LAN (WLAN) that is a technique that allows a user to wirelessly access the Internet by using a mobile terminal such as a personal digital assistant (PDA), a laptop computer, a potable multimedia player (PMP), etc. based on radio frequency technology at home or in companies or a specific service providing area.
While early WLAN technology supported a speed of 1 to 2 Mbps through frequency hopping, spread spectrum, infrared communication, etc. using a frequency of 2.4 GHz according to IEEE 802.11, recent WLAN technology has supported maximum 54 Mbps using orthogonal frequency division multiplex (OFDM). In addition, IEEE 802.11 standardizes various techniques such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access for vehicular environment, fast roaming, mesh network, interworking with external network, wireless network management, etc.
In IEEE 802.11, IEEE 802.11b uses a frequency of 2.4 GHZ and supports a communication speed of up to 11 Mbps. IEEE 802.11a commercialized after IEEE 802.11b uses a frequency of 5 GHz to reduce the influence of interference as compared to the frequency of 2.4 GHz and improves a communication speed to 54 Mbps by using OFDM. However, IEEE 802.11a has a shortcoming of a short communication distance as compared to IEEE 802.11b. Similarly to IEEE 802.11b, IEEE 802.11g uses 2.4 GHz to achieve a communication speed of 54 Mbps and meets backward compatibility, and thus it receives considerable attraction. Furthermore, IEEE 802.11g is superior to IEEE 802.11a in terms of the communication distance.
To overcome the limit of communication speed, which is a weak point of the WLAN, IEEE 802.11n has been recently established for the purpose of improving network speed and network reliability and extending wireless network operating distances. More specifically, IEEE 802.11n supports high throughput corresponding to a data processing rate of higher than 540 Mbps and is based on MIMO (Multiple Inputs Multiple Outputs) technique using multiple antennas for both a transmitter and a receiver to minimize a transmission error and optimize a data rate. Furthermore, IEEE 802.11n can use a coding method transmitting multiple repeated copies to increase data reliability and employ OFDM to increase a communication speed.
However, as WLAN has spread and applications using the WLAN have diversified, a new WLAN system for supporting throughputs higher than the by IEEE 802.11n throughput is thought to be desired. A very high throughput (VHT) WLAN system has been newly proposed in order to support a data processing rate of higher than 1 Gbps. The term VHT WLAN system is an arbitrary term. A feasibility test on a system using 4×4 MIMO and a channel bandwidth of higher than 80 MHz has been shown to provide a throughput of higher than 1 Gbps.
However, when a directional antenna is used in the IEEE 802.11 VHT system, hidden nodes (i.e., a node that cannot receive a contention free end (CF-End) frame) are frequently generated due to propagation characteristics (i.e., fading, scattering, directionality, etc). Hidden nodes do not receive the CF-End frame do not know that the the TXOP holder has transmitted the CF-End frame and thus released or truncated the TXOP. Accordingly, an improved TXOP truncation technique is required when the directional antenna is used.